Faceting for subplots in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is faceting in R's ggplot2?

Faceting is a way to split data into subsets and create multiple plots (subplots) arranged in a grid, each showing a subset of the data.

beginner

Which ggplot2 function is used to create subplots by rows and columns?

The function facet_grid() is used to create subplots arranged by rows and columns based on one or two variables.

intermediate

How does facet_wrap() differ from facet_grid()?

facet_wrap() creates subplots wrapped into multiple rows or columns based on one variable, while facet_grid() arranges subplots in a strict grid by two variables (rows and columns).

intermediate

What does the syntax facet_grid(rows ~ cols) mean?

It means subplots are arranged with the variable for rows on the left side and the variable for columns on the right side of the tilde (~). For example, facet_grid(Species ~ .) creates rows by Species and one column.

beginner

Why use faceting instead of multiple separate plots?

Faceting helps compare subsets side-by-side in one combined plot, making patterns easier to see and saving space compared to separate plots.

Which function in ggplot2 creates subplots arranged in a grid by two variables?

A geom_point()

B facet_wrap()

C facet_grid()

D ggplot()

facet_grid() arranges subplots in a grid by rows and columns based on two variables.

What does facet_wrap(~ variable) do?

A Creates subplots wrapped by one variable

B Creates a single plot with no subplots

C Creates subplots arranged by two variables

D Removes facets from the plot

facet_wrap() splits data by one variable and wraps subplots into rows or columns.

In facet_grid(rows ~ cols), what does the tilde (~) separate?

A X and Y axis variables

B Rows variable and columns variable

C Plot title and subtitle

D Data and aesthetics

The tilde separates the variable for rows (left) and columns (right) in the grid layout.

Which of these is a benefit of faceting?

A Hide data points

B Make the plot slower to render

C Change plot colors automatically

D Compare subsets side-by-side in one plot

Faceting helps visually compare different subsets easily by showing them side-by-side.

If you want subplots only by rows and no columns, which syntax is correct?

A facet_grid(variable ~ .)

B facet_grid(. ~ variable)

C facet_wrap(~ variable)

D facet_grid(variable)

facet_grid(variable ~ .) creates subplots arranged by rows only.

Explain how faceting works in ggplot2 and when you would use facet_wrap() versus facet_grid().

Think about how you want to arrange your subplots: wrapped or in a grid.

Describe the syntax and meaning of facet_grid(rows ~ cols) and give an example.

Remember the tilde (~) separates rows and columns variables.

Practice

1. What is the main purpose of faceting in R plotting with ggplot2?

easy

A. To save the plot as an image file

B. To change the color of points in a plot

C. To add titles to the plot

D. To split one plot into multiple smaller plots based on categories

Solution

1. Step 1: Understand faceting concept

   Faceting divides a single plot into multiple smaller plots based on categories in the data.

2. Step 2: Identify purpose in ggplot2

   In ggplot2, faceting helps compare groups by creating subplots for each category.

3. Final Answer:

   To split one plot into multiple smaller plots based on categories -> Option D

4. Quick Check:

   Faceting = split plot by category [OK]

Hint: Faceting means splitting plots by groups [OK]

Common Mistakes:

• Confusing faceting with coloring points
• Thinking faceting adds titles
• Believing faceting saves files

2. Which of the following is the correct syntax to create subplots by one variable using facet_wrap in ggplot2?

easy

A. ggplot(data) + geom_point() + facet_wrap(~ variable)

B. ggplot(data) + geom_point() + facet_grid(variable)

C. ggplot(data) + geom_point() + facet_wrap(variable)

D. ggplot(data) + geom_point() + facet_grid(~ variable)

Solution

1. Step 1: Recall syntax for facet_wrap

   facet_wrap requires a formula with tilde (~) and the variable name, like ~ variable.

2. Step 2: Check options

   ggplot(data) + geom_point() + facet_wrap(~ variable) uses facet_wrap(~ variable), which is correct syntax for faceting by one variable.

3. Final Answer:

   ggplot(data) + geom_point() + facet_wrap(~ variable) -> Option A

4. Quick Check:

   facet_wrap syntax = ~ variable [OK]

Hint: Use ~ before variable in facet_wrap() [OK]

Common Mistakes:

• Omitting the tilde (~) before variable
• Using facet_grid without formula
• Confusing facet_wrap and facet_grid syntax

3. What will be the result of this code?

library(ggplot2)
data <- data.frame(x=1:6, y=6:1, group=c('A','A','B','B','C','C'))
ggplot(data, aes(x, y)) + geom_point() + facet_wrap(~ group)

medium

A. A single plot with all points colored by group

B. Three separate plots, one for each group A, B, and C

C. An error because facet_wrap needs two variables

D. A plot with points connected by lines

Solution

1. Step 1: Understand data and faceting

   Data has groups A, B, and C. facet_wrap(~ group) creates one subplot per group.

2. Step 2: Predict output

   The code will produce three separate plots, each showing points for groups A, B, and C respectively.

3. Final Answer:

   Three separate plots, one for each group A, B, and C -> Option B

4. Quick Check:

   facet_wrap splits plot by group [OK]

Hint: facet_wrap(~ group) makes one plot per group [OK]

Common Mistakes:

• Expecting one combined plot with colors
• Thinking facet_wrap requires two variables
• Assuming points connect with lines

4. Identify the error in this code snippet:

ggplot(data, aes(x, y)) + geom_point() + facet_grid(group)

medium

A. facet_grid requires a formula with ~, like facet_grid(~ group)

B. geom_point() should come after facet_grid()

C. aes() must include color=group for faceting

D. facet_grid cannot be used with geom_point()

Solution

1. Step 1: Check facet_grid syntax

   facet_grid requires a formula input with tilde (~), e.g., facet_grid(~ group).

2. Step 2: Identify error in code

   The code uses facet_grid(group) without ~, causing a syntax error.

3. Final Answer:

   facet_grid requires a formula with ~, like facet_grid(~ group) -> Option A

4. Quick Check:

   facet_grid needs ~ before variable [OK]

Hint: Always use ~ in facet_grid() formula [OK]

Common Mistakes:

• Omitting ~ in facet_grid()
• Changing order of geom_point and facet_grid
• Adding unnecessary color aesthetic for faceting

5. You want to create a grid of plots faceted by two variables: gender (rows) and age_group (columns). Which code correctly does this in ggplot2?

hard

A. ggplot(data) + geom_point() + facet_wrap(~ gender + age_group)

B. ggplot(data) + geom_point() + facet_grid(~ gender + age_group)

C. ggplot(data) + geom_point() + facet_grid(gender ~ age_group)

D. ggplot(data) + geom_point() + facet_wrap(gender ~ age_group)

Solution

1. Step 1: Understand faceting by two variables

   facet_grid(rows ~ columns) creates a grid with rows and columns faceted by variables.

2. Step 2: Match variables to rows and columns

   gender should be rows, age_group columns, so syntax is facet_grid(gender ~ age_group).

3. Final Answer:

   ggplot(data) + geom_point() + facet_grid(gender ~ age_group) -> Option C

4. Quick Check:

   facet_grid(rows ~ columns) = gender ~ age_group [OK]

Hint: Use facet_grid(row ~ column) for two variables [OK]

Common Mistakes:

• Using facet_wrap for two variables as grid
• Swapping rows and columns in formula
• Omitting ~ or using wrong formula syntax